1. Field of the Invention
The invention concerns an inverter arrangement comprising two controlled three-phase inverters whose inputs are preferably connected to a common DC voltage source and whose outputs deliver a first and second three-phase system of output voltages of rectangular waveform which are displaced by 120.degree. electrical (hereinafter written as el) with respect to each other. More particularly, it concerns an inverter arrangement in which the aforesaid outputs of the two inverters are added together by means of a transformation circuit in transformer fashion to form a three-phase system of resultant output voltages, and in which at least one control unit is included for controlling the main valves of the two inverters such that all the inverter output voltages have a rectangular waveform.
2. Description of the Prior Art
The resultant output voltage of two signle-phase inverters which are connected to a common DC voltage source can be controlled by a known technique (see, e.g., Siemens-Zeitschrift October 1964, No. 10, pages 775 to 781, particularly FIG. 7 on page 779) which operates according to the principle of an electronic rotary transformer and is sometimes referred to as the tilt method. In this technique, the output voltages of both inverters are combined via two transformers connected in series on the secondary side to form a resultant output voltage whose magnitude can be changed by displacing in time the control pulses controlling the first inverter relative to the control pulses controlling the second inverter. More specifically, the output voltages of both inverters have the same amplitude and frequency and have a rectangular waveshape which includes per period a positive and a negative voltage pulse each of 180.degree. width. By displacing the control pulses of the first inverter relative to those of the second, a phase shift between the aforesaid two output voltages occurs. As a result, combining of the two output voltages advantageously results in a rectangular resultant output voltage which is approximately sinusoidal in shape and which has an amplitude dependent upon the phase shift between the combined voltages. This resultant output voltage is also found to have low-order harmonics which for may applications, e.g., for an interruption-free power supply, especially for feeding a data processing system, are highly undesirable. Moreover, with this manner of generation, the control speed is limited since it corresponds approximately to the duration of one half-period.
In another known technique an individual inverter is controlled employing the principles of pulse width modulation (Siemens-Zeitschrift 45 (1971), No. 3, pages 154 to 161). According to this technique, a pulsed three-phase inverter generates between its output terminals a symmetrial three-phase AC voltage system whose fundamental has a predetermined frequency and a controllable amplitude. The three output voltages of the inverter each have a rectangular waveform with a number of positive and negative voltage pulses per period. Moreover, each output voltage is approximately sinusoidal in shape and comprises, in addition to a fundamental, harmonics of different frequencies.
Since the aforesaid harmonics of the output voltage are undesirable, for instance, in the operation of a three-phase machine, as they result in current harmonics which impose an additional load on the machine, the number and position of the individual voltage pulses of the output voltages are selected so that the harmonics produced are those having a minimal detrimental effect. This, in turn, is achieved by selecting the position and number of pulses so as to eliminate all lower order harmonics. If this is done the harmonic currents are then kept small by the stray reactances present in the three-phase machine itself. One technique for realizing a high fundamental content and a low lower order harmonic content is by varying the pulse widths proportionally to the instantaneous value of the fundamental. Unfortunately, with this technique, the harmonics of low order generally cannot entirely be elminated if the individual output voltages are to be controllable.
It is, therefore, an object of the present invention to provide an inverter arrangement of the aforementioned type whose resultant output voltages each have a fundamental whose magnitude can change and an harmonic content which remains constant as to the order number of its harmonics during the changes in magnitude of its associated fundamental. In other words, each of the resultant output voltages is to be controllable and should have only harmonics above a given high order which do not change in order or number during the control process.